On commercial aircraft, there can be up to three very high frequency (VHF) communication radios which operate independently and simultaneously in multiple modes (with different modulation schemes) in the same designated aeronautical frequency band, such as 118-136.975 MHz. Often there is limited spatial separation between the antennae feeding the three VHF radios. The proximally located antennae are not isolated from each other. This lack of isolation between the antennae creates stringent requirements on the transmit side and the receive side of the transceivers in order to avoid interference from cross channel signals.
If there are spurious emissions and noise being transmitted from the transmitter end of the transceiver, the receive linearity and selectivity must be well controlled in order to prevent desensitization and to preserve the large dynamic range of the receiver. The speed and dynamic ranges of currently available Digital-to-Analog/Analog-to-Digital Converters (DAC/ADC) are not sufficient for direct digital up-conversion or down-conversion. Therefore, transceiver architectures are limited to complex, physically large, expensive, and power consuming technologies. Typically analog direct up conversion or heterodyne up-conversion/down-conversion technologies include filters, which are not the ideal for software-definable or cognitive radio systems.